In order to handle and connect a semiconductor die (integrated circuit device) to external systems, it is generally necessary to package the die. This usually involves mounting the die to some sort of substrate, leadframe or carrier, connecting bond pads on the die to some sort of conductive lines or traces and forming a package body around the die. The conductive lines or traces exit the package body, and usually terminate in external leads or pins.
For example, ceramic packages have a package body with a central opening (cavity) in one face for receiving the die, and lead fingers embedded in the body and extending into the opening. The die is connected (usually wire bonded) to the exposed (in the opening) portions of the lead fingers. The lead fingers are connected (internally in the package) to pins exiting a planar surface of the package. These pins are typically arranged in a rectangular (e.g., square) array. This type of packaging is expensive, and involves several elements for the ceramic package, not the least of which is the pins. In some instances, the die-receiving cavity is "up", on one face of the package body, and the pins are on the other, opposite face of the package body. In other instances, the die-receiving cavity is "down", on the same face of the package as the pins (in which case there are no pins in the area of the cavity). (The pins are deemed to be on the "bottom" of the ceramic body.)
In another example, the die is attached and connected (usually wire bonded) to a relatively rigid (thick) lead frame having several conductive lines, and a plastic body is molded around the die and inner ends of the conductive lead frame lines. Outer ends of the lead frame lines exit the plastic body on two or four sides. The leads may be closely spaced and delicate, demanding extreme care in handling to avoid lead skew and the like. Co-planarity (more correctly, lack thereof) is a problem with this, or any type of leaded (having external leads) package.
In another example, the die is attached to a relatively flexible (thin) lead frame supported by a plastic tape, and is encapsulated in epoxy or the like. Outer ends of the lead frame lines exit the epoxy body. These leads are even more fragile than those of plastic packages. Both wire bonding and tape-automated bonding of the die to the lead frame lines are well known in tape packages.
Another packaging technique is mounting the die directly to a printed circuit board (PCB; also referred to as PWB, or printed wiring board) substrate. The die is connected (typically wire bonded) to one end of conductive traces on a face (surface) of the PCB. Another end of the trace is connected to a pin, a separate external lead, or the like.
A recent development in semiconductor packaging is exemplified by Motorola's "OMPAC", or Overmolded Plastic Pad Array Carrier, whereby a semiconductor die is mounted face-up in a central area of a printed circuit board (PCB). The PCB is larger than the die. The top surface of the PCB is provided with a number of conductive traces that extend from near the periphery of the PCB to the central area. The die has bond pads on its face. Bond wires extend from these bond pads to inner ends of the traces. Near the periphery of the PCB, there are plated (conductive) through-holes (vias) extending from the back side of the PCB, through the PCB to a respective trace. The back side of the PCB is provided with a number of conductive traces, each having an end connected with a respective via. In this manner, signals (and power) to and from the die are connected through the bond wires, through the top side traces, through the vias, to the bottom side traces. A plastic molded body is formed over the die, and partially covers the top surface of the PCB.
Such "partial" coverage of the PCB by the molding compound results in a package body that can delaminate (separate) from the substrate (PCB) when the molded body shrinks after molding. Consequently, moisture can penetrate onto the chip along the delamination interface (plane between molded body and PCB) and cause reliability failures. The adhesion of the partially molded plastic body on to the substrate is generally considered as the weak link in this type of package, and is especially so when the plastic molded body gets large and the substrate is thin and flexible.
Further, in order to perform this type of "partial" molding, non-conventional molding equipment is required. The gate for injecting the mold compound cannot be at the parting line of the mold. The mold will thus have a cavity plate that needs to be removed from the mold press to unload the package. (The mold would be a three-part design with a cavity plate section that has to be removed from the mold to remove the completed package.)
Molding an entire package body over a die mounted to a leadframe ("fully molded") usually requires that the leadframe has a "dambar", namely a continuous ring of metal surrounding the body that prevents the plastic from flowing out of the mold cavity between the external leads of the leadframe. The dambar then has to be removed to isolate individual leads before the package is usable. With high pin count packages, the leads are often delicate and spaced closely (fine pitch), resulting in the need for very fine precision tooling for the trimming operation. This type of tooling is also very expensive, which adds to the overall cost of packaging.
Molding of plastic (or other suitable encapsulant) around the leadframe also causes some leakage of the plastic onto the leadframe (flashing). The flash then has to be removed in a separate de-flashing (dejunking) step.
Attention is directed to commonly-owned U.S. Pat. No. 5,051,813, entitled PLASTIC-PACKAGED SEMICONDUCTOR DEVICE HAVING LEAD SUPPORT AND ALIGNMENT STRUCTURE, which discloses plastic packaging with and without dambars, dejunking, etc.
In the main, hereinafter, molding where the mold gate is disposed at the parting plane of the two mold halves is discussed, as most pertinent to the present invention.
The following U.S. Pat. Nos. are cited of general interest in the field of packaging (annotations in parentheses): 3,405,441 (hermetic sealing process using glass and metal lid on a ceramic substrate); 3,909,838 (package formed by sealing two halves or pre-molded body around a molded pill package bonded to a leadframe); 4,143,456 (glob top sealing devices mounted on a substrate); 4,264,917 (silicon substrate with glob top encapsulation); 4,300,153 (TAB device with a substrate bonded to the bottom of the die; glob top encapsulation); 4,330,790 (tape-mounted device encapsulated using a metal carrier and epoxy); 4,363,076 (flat TAB assembly); 4,507,675 (molded heatsink package); 4,594,770 (bonding a metal cap and a plastic cap around a leadframe); 4,857,483 (mold gate is not located at the parting plane of the mold halves); 4,872,825 (encapsulation method using a lamination process instead of injection or transfer molding); 4,874,722 (pre-molded flatpack encapsulated with silicone gel; dambar required; not encapsulated by molding); 4,890,152 (molded pin grid array package; not a surface mount flatpack construction); 4,913,930 (coating and encapsulating a device in a reel-to-reel format); 4,955,132 (flip chip mounting to a substrate); 4,961,105 (die back metallization); 4,974,057 (die coated with resin and then molded); 4,975,765 (high density flatpack with edge connectors; not a molded package); 4,982,265 (stackable TAB); 4,984,059 (leadframe tips overlap the top of the die surface); 4,996,587 (thin,stackable package); and 5,025,114 (leadframe construction resulting in multilayer structure for plastic packages).